Multi-stage system, a control method therefor, and a lithographic apparatus

ABSTRACT

A multi-stage system includes a stator including a plurality of electric coils; a first stage including a first magnet assembly, the first stage moveable relative to the stator; a second stage including a second magnet assembly, the second stage moveable relative to the stator; a controller configured to position the first and the second stage relative to the stator by activating, respectively, a first subset of the plurality of electric coils to interact with the first magnet assembly and a second subset of the plurality of electric coils to interact with the second magnet assembly, the controller adapted to prevent at least one electric coil, to be simultaneously shared by the first and the second subset to position the first and the second stage on the stator, from activating.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/477,669, filed on May 22, 2012, which claims priority and benefitunder 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No.61/489,796, entitled “A Multi-Stage System, A Control Method Therefor,and A Lithographic Apparatus,” filed on May 25, 2011, the contents ofboth applications being incorporated herein in their entirety byreference.

FIELD

The present invention relates to a multi-stage system, a method tocontrol such a multi-stage system, and a lithographic apparatusincluding such a multi-stage system.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In such a case, a patterning device, which isalternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.including part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Conventional lithographicapparatus include so-called steppers, in which each target portion isirradiated by exposing an entire pattern onto the target portion atonce, and so-called scanners, in which each target portion is irradiatedby scanning the pattern through a radiation beam in a given direction(the “scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

To position an object, for instance the substrate table, it is common touse a so-called stage system. One type of stage system that is currentlyunder development is a single-stage system comprising a stator extendingsubstantially parallel to a first direction X and a second direction Y,wherein the second direction Y is perpendicular to the first directionX, and a first stage that is moveable relative to the stator in thefirst and second direction. A schematic example of such a single-stagesystem is shown in FIG. 2. The stator is indicated by the referencenumeral 1 and the first stage is indicated by reference numeral 3.

The first stage 3 is provided with a system of magnets. For simplicityreasons it can be assumed that in the example of FIG. 2 the entirebottom area of the first stage is occupied by the system of magnets. Thesystem of magnets generates a magnetic field that extends from thesystem of magnets to the stator underneath and in the vicinity of thefirst stage.

The stator is provided with an array of electric coils 5 of which only afew are indicated by reference numeral 5, the electric coils beingconfigured to interact with the magnetic field generated by the systemof magnets of the first stage in order to generate forces on the firststage to position them relative to the stator in the first and seconddirection.

It is noted here that the stator is usually mounted to or carried by aframe and thus acts as the stationary world. The first stage is able tomove relative to the stator. The stage system is thus of the movingmagnet type instead of the more commonly used moving coil type.

When basic control is used to position the stage relative to the stator,all coils on the stator are activated. However, with this configuration,most coils are not in the vicinity of the first stage with its system ofmagnets and thus have minimal interaction with the generated magneticfield. Furthermore, this does not allow for a second stage which canindependently be positioned relative to the stator using the same coils.

To avoid this, only a subset of coils is activated, so that only thecoils that have a non-negligible interaction with the magnetic field areactivated, where non-negligible can be determined by the requiredposition accuracy of the first stage. An example of a subset of coils isindicated in FIG. 2 by the bold coils 5. As shown in this example, onlythe coils directly under the first stage, i.e. the system of magnets,and in its direct vicinity are activated. The coils directly under thefirst stage are indicated by shading.

In FIG. 3, a multi-stage system is shown in which two stages, namely afirst stage 3 and a second stage 7, are moveable relative to a stator 1with multiple electric coils 5. To position the first stage 3, a firstsubset of electric coils is selected and activated as indicated by thebold coils. At the same time, the second stage can be positioned byselecting and activating a second subset of coils as indicated by thedashed coils.

A benefit of this configuration is that two stages can independently bepositioned at the same time with respect to the same stator. However,the two stages can not approach each other closely, which makes certaintypes of operations impossible to perform.

SUMMARY

It is desirable to provide a multi-stage system in which two stages areable to approach each other closely.

According to an embodiment of the invention, there is provided amulti-stage system comprising:

a stator extending substantially parallel to a first direction;

a first stage that is moveable relative to the stator in the first;

a second stage that is moveable relative to the stator in the first;

wherein the first and second stage are each provided with a system ofmagnets to generate a magnetic field,

wherein the stator is provided with a plurality of electric coils, saidelectric coils being configured to interact with the magnetic fieldsgenerated by the system of magnets of the first and second stage inorder to generate forces on the first and second stage to position themrelative to the stator,the multi-stage system further comprising:a sensor system to determine the position of the first and second stagerelative to the stator;a control unit to position the first and second stage relative to thestator in the first direction, wherein the control unit is configuredto:

determine the position of the first stage relative to the stator in thefirst direction based on an output of the sensor system;

select a first subset of electric coils that are capable of having anon-negligible interaction with the magnetic field of the system ofmagnets of the first stage in the determined position of the firststage;

determine the position of the second stage relative to the stator in thefirst direction based on an output of the sensor system;

select a second subset of electric coils that are capable of having anon-negligible interaction with the magnetic field of the system ofmagnets of the second stage in the determined position of the secondstage; and

activate the electric coils of the first and second subset in order toposition the first and second stage relative to the stator,

wherein the control unit is configured, prior to activating the electriccoils of the first and second subset, to:

determine the electric coils that are part of both the first and secondsubset; and

exclude at least one electric coil that is part of both the first andsecond subset from activating.

In another embodiment of the invention, there is provided a lithographicapparatus comprising a multi-stage system according to an embodiment ofthe invention.

In yet another embodiment of the invention, there is provided alithographic apparatus comprising a multi-stage system including:

a carrier extending substantially parallel to a first direction and asecond direction, wherein said second direction is perpendicular to thefirst direction;

a first stage that is moveable relative to the carrier in the first andsecond direction;

a second stage that is moveable relative to the carrier in the first andsecond direction;

wherein the first and second stage are each provided with a system ofmagnets to generate a magnetic field,

wherein the carrier is provided with an array of electric coils, saidelectric coils being configured to interact with the magnetic fieldsgenerated by the system of magnets of the first and second stage inorder to generate forces on the first and second stage to position themrelative to the carrier, the multi-stage system further comprising:a sensor system to determine the position of the first and second stagerelative to the carrier;a control unit to position the first and second stage relative to thecarrier in the first and second direction, wherein the control unit isconfigured to:

determine the position of the first stage relative to the carrier in thefirst and second direction based on an output of the sensor system;

select a first subset of electric coils that are capable of having anon-negligible interaction with the magnetic field of the system ofmagnets of the first stage in the determined position of the firststage;

determine the position of the second stage relative to the carrier inthe first and second direction based on an output of the sensor system;

select a second subset of electric coils that are capable of having anon-negligible interaction with the magnetic field of the system ofmagnets of the second stage in the determined position of the secondstage; and

activate the electric coils of the first and second subset in order toposition the first and second stage relative to the carrier,

wherein the control unit is configured, prior to activating the electriccoils of the first and second subset, to:

determine the electric coils that are part of both the first and secondsubset; and

exclude at least one electric coil that is part of both the first andsecond subset from activating, and wherein the lithographic apparatusfurther comprises:

an illumination system configured to condition a radiation beam;

a support constructed to support a patterning device, the patterningdevice being capable of imparting the radiation beam with a pattern inits cross-section to form a patterned radiation beam;

a first and second substrate table, each constructed to hold asubstrate; and

a projection system configured to project the patterned radiation beamonto a target portion of the substrate,

wherein the first substrate table is provided on the first stage and thesecond substrate table is provided on the second stage, such that thefirst and second substrate table can be positioned by appropriatepositioning of the first and second stage.

In a further embodiment of the invention, there is provided a method forcontrolling

a multi-stage system comprising:

a stator extending substantially parallel to a first direction;

a first stage that is moveable relative to the stator in the firstdirection;

a second stage that is moveable relative to the stator in the firstdirection;

wherein the first and second stage are each provided with a system ofmagnets to generate a magnetic field, and

wherein the stator is provided with a plurality of electric coils, saidelectric coils being configured to interact with the magnetic fieldsgenerated by the system of magnets of the first and second stage inorder to generate forces on the first and second stage to position themrelative to the stator in the first direction,said method comprising:

-   -   determining the position of the first stage relative to the        stator in the first direction;    -   selecting a first subset of electric coils that are capable of        having a non-negligible interaction with the magnetic field of        the system of magnets of the first stage in the determined        position of the first stage;    -   determining the position of the second stage relative to the        stator in the first direction;    -   selecting a second subset of electric coils that are capable of        having a non-negligible interaction with the magnetic field of        the system of magnets of the second stage in the determined        position of the second stage; and    -   activating the electric coils of the first and second subset in        order to position the first and second stage relative to the        stator,        wherein activating the electric coils of the first and second        subset comprises:    -   determining the electric coils that are part of both the first        and second subset; and    -   excluding at least one electric coil that is part of both the        first and second subset from activating.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIG. 2 depicts a conventional single-stage system;

FIG. 3 depicts a multi-stage system according to an embodiment of theinvention;

FIG. 4 depicts the multi-stage system of FIG. 3 with the stagesapproaching each other;

FIG. 5 depicts the multi-stage system of FIG. 3 where coils are excludedfrom activation to allow the approach of the stages towards each other;

FIG. 6 depicts the multi-stage system of FIG. 3, wherein the stagescontact each other;

FIG. 7 depicts the multi-stage system of FIG. 3, wherein after makingcontact the two stages are controlled as being a larger single stage;

FIG. 8 depicts in more detail the system of magnets of two stages of amulti-stage system according to another embodiment of the invention;

FIG. 9 depicts an assembly to control a multi-stage system in accordancewith an embodiment of the invention; and

FIG. 10 depicts a multi-stage system in accordance with an embodiment ofthe invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus includes an illuminationsystem (illuminator) IL configured to condition a radiation beam B (e.g.UV radiation or any other suitable radiation), a patterning devicesupport or mask support structure (e.g. a mask table) MT constructed tosupport a patterning device (e.g. a mask) MA and connected to a firstpositioning device PM configured to accurately position the patterningdevice in accordance with certain parameters. The apparatus alsoincludes a substrate table (e.g. a wafer table) WT or “substratesupport” constructed to hold a substrate (e.g. a resist-coated wafer) Wand connected to a second positioning device PW configured to accuratelyposition the substrate in accordance with certain parameters. Theapparatus further includes a projection system (e.g. a refractiveprojection lens system) PS configured to project a pattern imparted tothe radiation beam B by patterning device MA onto a target portion C(e.g. including one or more dies) of the substrate W,

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device,”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section so as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables or “substrate supports” (and/or two or more masktables or “mask supports”). In such “multiple stage” machines theadditional tables or supports may be used in parallel, or preparatorysteps may be carried out on one or more tables or supports while one ormore other tables or supports are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the patterning device (e.g. mask) and the projection system.Immersion techniques can be used to increase the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that a liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD configured to adjust theangular intensity distribution of the radiation beam. Generally, atleast the outer and/or inner radial extent (commonly referred to asσ-outer and σ-inner, respectively) of the intensity distribution in apupil plane of the illuminator can be adjusted. In addition, theilluminator IL may include various other components, such as anintegrator IN and a condenser CO. The illuminator may be used tocondition the radiation beam, to have a desired uniformity and intensitydistribution in its cross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask table)MT, and is patterned by the patterning device. Having traversed thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PS, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioning device PW andposition sensor IF (e.g. an interferometric device, linear encoder orcapacitive sensor), the substrate table WT can be moved accurately, e.g.so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioning device PM and anotherposition sensor (which is not explicitly depicted in FIG. 1) can be usedto accurately position the patterning device (e.g. mask) MA with respectto the path of the radiation beam B, e.g. after mechanical retrievalfrom a mask library, or during a scan. In general, movement of thepatterning device support (e.g. mask table) MT may be realized with theaid of a long-stroke module (coarse positioning) and a short-strokemodule (fine positioning), which form part of the first positioningdevice PM. Similarly, movement of the substrate table WT or “substratesupport” may be realized using a long-stroke module and a short-strokemodule, which form part of the second positioner PW. In the case of astepper (as opposed to a scanner) the patterning device support (e.g.mask table) MT may be connected to a short-stroke actuator only, or maybe fixed. Patterning device (e.g. mask) MA and substrate W may bealigned using patterning device alignment marks M1, M2 and substratealignment marks P1, P2. Although the substrate alignment marks asillustrated occupy dedicated target portions, they may be located inspaces between target portions (these are known as scribe-lane alignmentmarks). Similarly, in situations in which more than one die is providedon the patterning device (e.g. mask) MA, the patterning device alignmentmarks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g. mask table) MT or“mask support” and the substrate table WT or “substrate support” arekept essentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT or “substrate support”is then shifted in the X and/or Y direction so that a different targetportion C can be exposed. In step mode, the maximum size of the exposurefield limits the size of the target portion C imaged in a single staticexposure.2. In scan mode, the patterning device support (e.g. mask table) MT or“mask support” and the substrate table WT or “substrate support” arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT or “substrate support”relative to the patterning device support (e.g. mask table) MT or “masksupport” may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.3. In another mode, the patterning device support (e.g. mask table) MTor “mask support” is kept essentially stationary holding a programmablepatterning device, and the substrate table WT or “substrate support” ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or“substrate support” or in between successive radiation pulses during ascan. This mode of operation can be readily applied to masklesslithography that utilizes programmable patterning device, such as aprogrammable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Although FIG. 1 depicts only one positioning device PW, the lithographicapparatus comprises two and possibly more of such positioning devices PWin order to position respective substrate tables WT with respect to aframe FR. The positioning devices PW may alternatively be referred to asstages. Alternatively or additionally, other positioning devices such asthe positioning device PM for the patterning device support (e.g. masktable) MT may be depicted as a single stage, where in fact more of suchstages are present.

The frame FR of the lithographic apparatus comprises a stator 1 of whicha schematic example is shown in FIG. 3. The stator 1 extends in a firstdirection X and in a second direction Y (see FIG. 1). The stator isprovided with multiple electric coils 5 of which only a few areindicated by a corresponding reference numeral. The electric coils 5 arethus stationary mounted to the frame FR.

A first stage 3 is depicted in FIG. 3 to represent one of thepositioning devices PW of FIG. 1. The first stage is schematically shownand transparent to show the underlying electric coils which areindicated by shading. The first stage 3 is moveable relative to thestator 1 in both the first and second direction X,Y, and comprises afirst system of magnets to generate a first magnetic field. Although thesystem of magnets is not shown in FIG. 3 for simplicity reasons, it canbe assumed that the outer contour of the first stage 3 in FIG. 3 is alsothe outer contour of the system of magnets and that thus the firstmagnetic field also extends to outside the first stage.

Forces can be applied to the first stage 3 due to interaction betweenthe electric coils 5 and the first magnetic field, where thecontribution of each electric coil to the forces depends on the distanceand orientation to the first magnetic field.

FIG. 3 also depicts a second stage 7 to represent another one of thepositioning devices PW of FIG. 1. Similar to the first stage 3, thesecond stage is moveable relative to the stator 1 in both the first andsecond direction X, Y, and comprises a second system of magnets togenerate a second magnetic field.

Forces can be applied to the second stage 7 due to interaction betweenthe electric coils 5 and the second magnetic field, where thecontribution of each electric coil to the forces depends on the distanceand orientation to the second magnetic field. In this embodiment, thefirst stage and second stage are identical and may thus be interchanged.

To control the multi-stage system of FIG. 3, the generic andschematically depicted control scheme of FIG. 9 is used. The controlscheme depicts the first stage 3, the second stage 7, and the stator 1with the electric coils 5 from the side. The position in the first andsecond direction is measured by a sensor system that in this embodimentis formed by two interferometers IF, but the sensor system may alsocomprise for instance encoders, or a combination of encoders andinterferometers. The output of the two interferometers is provided to acontrol unit or controller CU. It is to be noted that the position ofthe first and second stage may also be measured by a sensor systemincluding multiple sensors, where the output of the multiple sensors arecombined to yield the position of the first and second stage relative tothe stator. This may for instance be the case when the stator ismoveable relative to the frame and the positions of the first and secondstage are measured relative to the frame. To determine the positions ofthe first and second stage relative to the stator, the position of thestator relative to the frame also has to be determined and has to becombined with the positions of the first and second stage relative tothe frame.

The control unit or controller is configured to:

-   -   determine the position of the first stage 3 relative to the        stator 1 in the first and second direction based on the output        of the sensor system;    -   select a first subset of electric coils 5 that are capable of        having a non-negligible interaction with the first magnetic        field of the first system of magnets of the first stage 3 in the        determined position of the first stage 3;    -   determine the position of the second stage 7 relative to the        stator 1 in the first and second direction based on the output        of the sensor system;    -   select a second subset of electric coils 5 that are capable of        having a non-negligible interaction with the second magnetic        field of the second system of magnets of the second stage 7 in        the determined position of the second stage 7;    -   activate the electric coils of the first and second subset using        the drive signal DS in order to position the first and second        stage 3,7 relative to the stator 1,        wherein the control unit or controller CU is further configured,        prior to activating the electric coils of the first and second        subset, to determine the electric coils that are part of both        the first and second subset and to exclude at least one electric        coil that is part of both the first and second subset from        activating.

In the example of FIG. 3 there is no overlap between first and secondsubset so that all electric coils can be activated by the control unitor controller CU. The selected and activated coils of the first subsetare indicated by bold lines and the selected and activated coils of thesecond subset are indicated by dashed lines.

The non-negligible interaction between coils and the respective magneticfields is determined in an embodiment by the required position accuracyand/or by the amount of force required. For both requirements it holdsthat the further away the electric coils are from a magnetic field, thesmaller the contribution to meet these requirements is.

In the example of FIG. 3 all electric coils can be activated for thegiven positions of the first and second stage. Therefore, the first andsecond stage can independently be positioned with respect to the statorwith the required position accuracy and/or amount of force. However,when the two stages approach each other, for instance when they have toperform close to each other or they have to pass each other in limitedspace, a coil of the first subset may also interact with the secondmagnetic field and a coil of the second subset may also interact withthe first magnetic field. Due to this disturbance and due to the factthat a coil can only be activated for positioning one stage and not forpositioning two stages at the same time, the first and second stage cannot approach each other if the normal control scheme is not changed. Thecontrol unit or controller of the multi-stage system according to anembodiment is able to co cope with this problem by excluding at leastone coil that is part of both the first and second subset, therebyeliminating the disturbance to the other stage of this coil, whichallows the first and second stage to approach each other more closelythan in prior art multi-stage systems. Additional explanation will begiven by reference to the FIGS. 4-6.

FIG. 4 depicts the multi-stage system of FIG. 3 in a situation where thefirst stage 3 and the second stage 7 have approached each other as closeas is possible using all coils of the first and second subset. In thenext step, first stage 3 remains stationary relative to the stator 1 andthe second stage 7 is moved towards first stage 3. At a certain momentthe required second subset of electric coils should include the rightmost column of electric coils of the first subset in order to positionthe second stage. However, as this will also affect the position of thefirst stage, these electric coils are excluded from activation as shownin FIG. 5.

In FIG. 6 even more electric coils have been excluded from activation inorder to let the two stages touch each other.

In an embodiment, the controller may be configured such that when thetwo stages get into a certain distance from each other such that thefirst and second subset overlap, only the electric coils directly andcompletely beneath the corresponding system of magnets are activated. Anexample of this is shown with reference to FIG. 10 in which the twostages of FIG. 3 pass each other (in the by the arrows indicateddirections) and during passing the first and second subset determined bythe control unit overlap as a result of which only the coils directlyand completely beneath the respective stages are activated by thecontrol unit.

While controlling the multi-stage system, the control unit may furtherbe configured to always ensure that a minimum amount of electric coils,e.g. nine electric coils, is activated in order to ensure that fullcontrol possibilities in all degrees of freedom are still available.

FIG. 7 depicts the situation of FIG. 6 in which the two stages havecontacted each other, but in this embodiment the control unit isconfigured to position the two stages as a single stage by activatingall electric coils of the first and second subset as indicated by thesolid coils in FIG. 7, where coils beneath the first and second stageare also activated and indicated by shading again. The first and secondstage will as a result of this move together. When required, the controlunit may further be configured to add a small current to the coils suchthat and additional force is generated that push the two stages againsteach other thereby ensuring that they remain in contact. This may forinstance be beneficial when the lithographic apparatus is of the type inwhich an immersion liquid is present between the projection system and asubstrate table supported by the first or second stage and both thefirst and second stage move under the projection system in order totransfer the immersion liquid from one substrate table to the othersubstrate table which requires close contact between the two stagescarrying the respective substrate tables.

To allow the situation of FIG. 7, the system of magnets may need to beadapted to this situation. In FIG. 8 the first stage 3 and the secondstage 7 are shown from below so that the respective first and secondsystem of magnets become visible.

Both system of magnets include first magnets 9 having a magnetizationdirection substantially perpendicular to the stator and directed towardsthe stator and second magnets 11 having a magnetization directionsubstantially perpendicular to the stator and directed away from thestator, the first and second magnets being arranged in accordance with apattern of rows and columns substantially perpendicular thereto, suchthat the first and second magnets are arranged in each row and in eachcolumn alternately, and wherein the two stages are positionable tocontact each other such that the pattern of the system of magnets of thefirst stage continues in the system of magnets of the second stage asshown in FIG. 8.

The system of magnets of FIG. 8 also have an odd number of rows and anodd number of columns. Further, the system of magnets of the first stage3 has first magnets on the diagonals and the system of magnets of thesecond stage 7 has second magnets on the diagonals. Due to this, thestages can be rotated about a rotation axis that is perpendicular to thefirst and second direction without altering the pattern of the system ofmagnets compared to the pattern of the other system of magnets so thatthe stages can contact each other from each side.

It is specifically noted here that the multi-stage system is not limitedby the number of degrees of freedom that can be controlled by thecontrol unit, but usually the number of degrees of freedom that arecontrolled lies between 3 and 6. In case of 6 degrees of freedom perstage, this requires a minimum of nine coils to have full 6 degree offreedom control available.

It will be apparent to the skilled person that although not mentioned,the interaction between the coils and magnetic field can also be used togenerate forces in a third direction perpendicular to both the first andsecond direction, so that a stage can also be elevated and positioned insaid direction. It will further be apparent that the same principles canbe applied to a system in which the first and second stage are onlymoveable relative to the stator in a first direction.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

What is claimed is:
 1. A multi-stage system comprising: a statorincluding a plurality of electric coils; a first stage including a firstmagnet assembly, the first stage moveable relative to the stator; asecond stage including a second magnet assembly, the second stagemoveable relative to the stator; a controller configured to position thefirst and the second stage relative to the stator by activating,respectively, a first subset of the plurality of electric coils tointeract with the first magnet assembly and a second subset of theplurality of electric coils to interact with the second magnet assembly,said controller adapted to prevent at least one electric coil, to besimultaneously shared by the first and the second subset to position thefirst and the second stage on the stator, from activating.
 2. Themulti-stage system according to claim 1, wherein prior to activating thefirst and the second subset of the plurality of electric coils, thecontroller is configured to: determine the electric coils that are partof both the first and the second subset; and prevent the at least oneelectric coil, to be simultaneously shared by the first and second thesubset to position the first and the second stage on the stator, fromactivating.
 3. The multi-stage system according to claim 1, comprising asensor configured to determine the position of the first and the secondstage relative to the stator, the sensor in communication with thecontroller.
 4. The multi-stage system according to claim 3, wherein thecontroller is configured to determine the position of the first stagerelative to the stator based on an output of the sensor and to selectthe first subset of the plurality of electric coils capable of having anon-negligible interaction with a magnetic field of the first magnetassembly in the determined position of the first stage.
 5. Themulti-stage system according to claim 3, wherein the controller isconfigured to determine the position of the second stage relative to thestator based on an output of the sensor and to select the second subsetof the plurality of electric coils capable of having a non-negligibleinteraction with a magnetic field of the second magnet assembly in thedetermined position of the second stage.
 6. The multi-stage systemaccording to claim 1, wherein the first subset is selected based on arequired positioning accuracy that is to be obtained for the firststage, and wherein the second subset is selected based on a requiredpositioning accuracy that is to be obtained for the second stage.
 7. Themulti-stage system according to claim 1, wherein at least the electriccoils that are directly and completely beneath the respective first andsecond magnet assemblies are activated during the positioning of thefirst and the second stage.
 8. The multi-stage system according to claim1, wherein at least nine coils of the first subset and at least ninecoils of the second subset are activated during the positioning of thefirst and the second stage.
 9. The multi-stage system according to claim1, wherein the controller is configured to allow the first and secondstages to approach each other by preventing electric coils positionedbetween the first and the second stage from activating.
 10. Themulti-stage system according to claim 9, wherein the controller isconfigured to control the first and the second stage as a single stagewhen the first and the second stage have approached and contacted eachother by activating all electric coils of the first and the secondsubset.
 11. The multi-stage system according to claim 10, wherein eachof the first and second magnet assemblies includes first magnets havinga magnetization direction substantially perpendicular to a carrier anddirected towards the carrier and second magnets having a magnetizationdirection substantially perpendicular to the carrier and directed awayfrom the carrier, the first and second magnets being arranged inaccordance with a pattern of rows and columns substantiallyperpendicular thereto, such that the first and second magnets arearranged in each row and in each column alternately, and wherein the twostages are positionable such that when the first and the second stagecontact each other the pattern of the system of magnets of the firststage continues in the system of magnets of the second stage.
 12. Themulti-stage system according to claim 11, wherein each of the first andthe second magnet assemblies has an odd number of rows and an odd numberof columns, and wherein the first magnet assembly has first magnetsarranged on the diagonals and the second magnet assembly has secondmagnets arranged on the diagonals.
 13. The multi-stage system accordingto claim 1, wherein the stator defines a plane on which the first andsecond stages are movable.
 14. A lithographic apparatus comprising: asupport constructed to support a patterning device, the patterningdevice being capable of imparting a radiation beam with a pattern in itscross-section to form a patterned radiation beam; a first and a secondobject table; and a projection system configured to project thepatterned radiation beam onto a target portion of a substrate, and amulti-stage system comprising: a stator including a plurality ofelectric coils; a first stage including a first magnet assembly, thefirst stage moveable relative to the stator; a second stage including asecond magnet assembly, the second stage moveable relative to thestator; a controller configured to position the first and the secondstage relative to the stator by activating, respectively, a first subsetof the plurality of electric coils to interact with the first magnetassembly and a second subset of the plurality of electric coils tointeract with the second magnet assembly, said controller adapted toprevent at least one electric coil, to be simultaneously shared by thefirst and the second subset to position the first and the second stageon the stator, from activating, wherein the first object table isprovided on the first stage and the second object table is provided onthe second stage, such that the first and second object table can bepositioned by appropriate positioning of the first and second stage. 15.The lithographic apparatus according to claim 14, wherein prior toactivating the first and the second subset of the plurality of electriccoils, the controller is configured to: determine the electric coilsthat are part of both the first and the second subset; and prevent theat least one electric coil, to be simultaneously shared by the first andthe second subset to position the first and the second stage on thestator, from activating.
 16. The lithographic apparatus according toclaim 14, comprising a sensor configured to determine the position ofthe first and the second stage relative to the stator, the sensor incommunication with the controller.
 17. The lithographic apparatusaccording to claim 16, wherein the controller is configured to determinethe position of the first stage relative to the stator based on anoutput of the sensor and to select the first subset of the plurality ofelectric coils capable of having a non-negligible interaction with amagnetic field of the first magnet assembly in the determined positionof the first stage.
 18. The lithographic apparatus according to claim16, wherein the controller is configured to determine the position ofthe second stage relative to the stator based on an output of the sensorand to select the second subset of the plurality of electric coilscapable of having a non-negligible interaction with a magnetic field ofthe second magnet assembly in the determined position of the secondstage.
 19. The lithographic apparatus according to claim 14, wherein thefirst subset is selected based on a required positioning accuracy thatis to be obtained for the first stage, and wherein the second subset isselected based on a required positioning accuracy that is to be obtainedfor the second stage.
 20. The lithographic apparatus according to claim14, wherein at least the electric coils that are directly and completelybeneath the respective first and second magnet assemblies are activatedduring the positioning of the first and the second stage.